I had the same problems with interlacing, but even uglier, terrible ghosting on all moving objects (look at the bat/stick on your .png!) while rendering some 50i footage to 25p.
I did a lot of searching, but there was almost nothing to find about it. I guess it's some specific HV20 PAL in Vegas problem. I don't have the same problems with my XHA1 footage captured through the HV20 as a deck.

Anyway, i found a solution: 'right-click' each clip and check the 'disable resample' box.
It did work for me in combination with 'interpolate fields', but that's because i was working with 50i footage. It can work for you to maybe.